Article made of a magnesium alloy tube

ABSTRACT

An article made of a magnesium alloy tube ( 23 ) is provided, the article having a grain size of between 10 μm and 50 μm and being manufactured by internal high pressure forming. The temperature of the internal high pressure forming is between 200° C. and 605° C. The tube ( 23 ) is manufactured by extrusion, wherein the extrusion temperature is between 300° C. and 605° C., extrusion speed is substantially between 5 mm/sec and 45 mm/sec, and the extrusion reduction ratio is substantially between 10:1 and 50:1. The tube ( 23 ) may be annealed for 6 hours at 300° C.

FIELD OF THE INVENTION

This invention relates to processes for extrusion of tubes from metalalloys and structures formed by such processes, in particular toextrusion using magnesium alloys.

BACKGROUND OF THE INVENTION

Metal profiles and tubes are often produced using extrusion. In theprocess, a round metal block called a “billet” is introduced into aheated container, having a ram at one end and a die at the other. Theram is used to apply a pressure on the billet in the direction of thedie. The billet is forced through an aperture in the die, and the metalundergoes a deformation into an extruded body, having a cross-section inthe shape of the aperture. This is referred to as direct extrusion. Inanother known method of extrusion, the die is moved toward the billet.The billet therefore is stationary with respect to the container,eliminating friction there and reducing the pressure needed forextrusion. However, surface defects on the billet are propagated to theextruded material. This is referred to indirect extrusion.

The deformation of an object with one cross-sectional area into anobject of a smaller cross-sectional area results in an increase inlength. The ratio of the cross-sectional area of the billet to thecross-sectional area of the extruded body is referred to in the art aswell as in the present specification and claims, as the extrusionreduction ratio.

In order to create hollow shapes such as tubes using extrusion, amandrel is placed inside the die aperture. The metal is forced to flowbetween the die and the mandrel, causing a hollow having the shape ofthe mandrel to be formed in the metal.

Recently, internal high pressure forming (IHPF) has been utilized toform parts having complex geometries, particularly in the automotiveindustry. IHPF utilizes fluid pressure to form the part into the desiredshape of a mold. Using this process, components that would otherwise bemanufactured as several pieces and then joined together can bemanufactured as one. This reduces the mass of the component andmaintains its stiffness, due to the elimination of spot-welded joints.

Other advantages of using IHPF include weight reduction through moreefficient section design of the component and tailoring of its wallthickness, improved structural strength, lower tooling cost due to fewerparts, fewer secondary operations such as welding of sections, tighterdimensional tolerances, and fewer parts required for final assembly.

Presently, IHPF is used to manufacture steel and aluminium parts.

SUMMARY OF THE INVENTION

The present invention is directed toward adaptation of internal highpressure forming to magnesium, for use, in particular, in reducing avehicle's weight and to thereby increase fuel economy and reduceemissions.

According to one aspect of the present invention, there is provided anarticle made of a magnesium alloy tube, the article having a grain sizeof between 10 μm and 50 μm and being manufactured by internal highpressure forming. The temperature of the internal high pressure formingis between 200° C. and 605° C. The tube is manufactured by extrusion,wherein the extrusion temperature is between 300° C. and 605° C., theextrusion speed is substantially between 5 mm/sec and 45 mm/sec, and theextrusion reduction ratio is substantially between 10:1 and 50:1. Thetube may be annealed for 6 hours at 300° C.

According to another aspect of the present invention, there is provideda process for forming a hollow tube by extrusion of a billet made from amagnesium alloy, comprising the steps of heating the billet to apredetermined temperature within a temperature range, extruding thebillet using an indirect extrusion process, wherein the temperature iswithin a range of 300° C. to 605° C., the extrusion speed issubstantially in the range between 5 mm/sec and 45 mm/sec, and theextrusion reduction ratio is substantially in the range between 10:1 and50:1.

According to a further aspect of the invention, there is provided aprocess for internal high pressure forming using tubes obtainedaccording to the above-described process. The process comprises thesteps of cooling the tube in a predetermined temperature for apredetermined amount of time, sealing the tube from both ends,introducing a pressure medium into the tube, positioning the tube in amold having a guiding zone and an expansion zone, wherein thetemperature of the expansion zone is between 200° C. and 605° C., andapplying an axial compression force on the tube, the result being thatthe portion of the tube that is within the expansion zone expands tofill the zone and adopt thereby its shape. The tube is then cooled for apredetermined period of time at a predetermined temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, an embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 shows a partial cut-away view of a typical extrusion press whichmay be used with an extrusion process of the present invention;

FIG. 2 is an illustration of a typical setup for a tooling which may beused in internal high pressure forming; and

FIG. 3 is an illustration the setup of FIG. 2, wherein a tube isundergoing internal high pressure forming.

DETAILED DESCRIPTION OF THE INVENTION

An extrusion press, known in the art, is employed using specificparameters in order to form extruded magnesium parts with propertiesthat enable plastic deformation to be accomplished thereon in industrialapplications, such as in the automotive industry.

FIG. 1 illustrates an extrusion press 10 comprising a container 12adapted to receive a magnesium billet 14, a ram 16, a die 18, and afloating mandrel (not seen in FIG. 1). The die 18 comprises an aperture19 of a predetermined shape. The magnesium billet 14 is heated so thatits temperature is within a range of 300° C. to 605° C. The heatedbillet 14 is positioned in the container 12, and the die 18 is movedtoward the billet 14, exerting a pressure which forces the billet 14 todeform, becoming an extrusion 20 which has a circumference similar tothe aperture 19. The extrusion press 10 is adapted to maintain thebillet 14 at the predetermined temperature during extrusion. The die 18is moved at an extrusion rate substantially between 5 mm/sec and 45mm/sec. The mandrel is positioned so that it forms a hollow 21 in theextrusion 20 and determines the size and shape of the hollow. The spacebetween the die 18 and the mandrel determines the cross-sectional areaof the extrusion 20. The extrusion reduction ratio is substantiallybetween 10:1 and 50:1.

When heating the magnesium billet 14, it should be kept in mind thatadditional heating will occur due to the pressure inherent to theextrusion process. Therefore, the billet 14 should not initially beheated to the desired temperature, so that it is not exceeded during theextrusion.

The billet 14 used for the extrusion may be an undrilled billet, or apredrilled billet having a hollow center.

The magnesium used in this process may be any commercially availablemagnesium alloy, such as AM60, AS41, AZ31, AZ61, AZ80, AZ91, ZE41, orZM21. Alternatively, it may be a custom-made alloy. Preferably, themagnesium used with the process of the present invention is either theAZ31 or ZM21 alloy. The combination of parameters for any givenmagnesium alloy may be determined by one skilled in the art by trial anderror.

According to one particular embodiment, the chemical composition of themagnesium alloy used in the process according to the present inventionis 2.856% Al, 1.022% Zn, 0.329% Mn, 0.004% Fe, 0.038% Si, 0.001% Cu, and0.001% Ni. In one experiment performed, a continuous casting system wasused with a flat die of 42 mm, a mandrel of 38 mm and a container of 110mm. The magnesium billet was 155 mm in length, 107 mm in diameter andwas predrilled with a hole diameter of 38 mm (need to confirmpre-drilled). The extrusion rate was 1.6 mm/sec, with a billet andcontainer heated to 320° C. The extrusion reduction ratio was about 30:1with an overall force of 4.45 MN.

The magnesium tube obtained using the above-described process hadmechanical properties which provide the strength and durability towithstand, without failure, plastic deformations inherent in furtherproduction involving the tube. Specifically, it had a yield strength of195 N/mm², an ultimate tensile strength (UTS) of 255 N/mm², and a 14%elongation at fracture.

One process for which the magnesium tube obtained as described above maybe used is internal high pressure forming. With reference to FIG. 2,this is accomplished with the aid of a specialized tooling 22. Thetooling 22 comprises a clamping unit 25 disposed on each side of thetube 23 and a forming unit 27 surrounding it. The clamping unit 25comprises crossheads 24, hydraulic cylinders 26, sealing punches 28, andappropriate heating and cooling means. The sealing punches 28 areadapted to seal the ends of the tube during forming, and at least onecomprises an opening 42 adapted to introduce a pressure medium from apressure source (not shown) into the tube. The forming unit 27 comprisesbody material 30, dies 32 which form an expansion zone 36, inserts 34which form guiding zones 38, and appropriate heating and cooling means.

Referring to FIG. 3, the tube 23 is deformed within the tooling 22. Whenthe tube 23 is retained in the tooling, its ends are sealed, and apressure medium is introduced at a forming pressure therein through theopening 42. A high pressure is applied to the medium, and an axialcompression force (indicated by arrows 44) is applied to the tube 23.The expansion zone 36 is maintained at a temperature, substantially inthe range of 200° C. to 605° C., to permit the necessary circumferentialstrain for forming. In the guiding zone 38 a lower temperature ismaintained, for example, in the range of 100° C. to 200° C. This is toensure that the flow stress of the portion of the tube 23 in the guidingzone 38 is sufficient to transfer the axial force to the portion of thetune in the expansion zone 36. (For example, depending on the alloyused, in the temperature range of about 350° C. to 400° C., the flowstress can decrease to 40 N/mm². The higher the axial force that istransferred to the portion of the tube in the expansion zone, the higherthe circumferential strain that can be achieved, which leads to higherformability.) As a result, the tube 23 deforms in the expansion zone 36,and forms a bulge 40 which takes the shape of the die 32.

It will be appreciated to one skilled in the art that the formingpressure is determined based on the magnesium alloy used to manufacturethe tube, the thickness thereof, the specific geometry of the part to beformed, and the temperature used for forming. A smaller radius and alower temperature require a higher forming pressure. In addition, athicker wall requires a higher forming pressure.

The pressure medium is a fluid, being either a gas or a heat resistantfluid. According to one embodiment, it is a gas which does not reactwith any other material it will come in contact with during the courseof IHPF. It is preferably recoverable for subsequent use. Specifically,nitrogen or one of the noble gasses is used. Using a gas has severaladvantages over using a liquid, in that gasses may be heated to hightemperatures as described with respect to the processes herein withoutundergoing decomposition.

The formability of the magnesium tube during IHPF is partiallyinfluenced by the strain rate of the material. This influence increaseswith higher temperatures. Depending on process time and necessaryformability, the strain rate may vary between 2×10⁻¹ sec⁻¹and 1×10⁻⁴sec⁻¹.

Previous heat treatment may also influence the initial structure of themagnesium tube. The tubes may be annealed at temperatures that aresubstantially between 200° C. and 600° C. In one preferred case, theannealing temperature is between 250° C. and 300° C. When annealed inthis temperature range for a maximum of 6 hours, a uniform, fine-grainedstructure may be observed in the tube. A grain size between 10 μm and 50μm can be realized. This fine-grained structure leads to an improvedformability, especially at low strain rates (smaller then 2×10⁻¹ sec⁻¹).

The process described herein, and more specifically, the steps whichinvolve IHPF may additionally be followed by subsequent IHPF operationsin order to further deform or provide additional structuralmodifications to the tube.

According to one example, the following conditions are used for IHPF.Subsequent to extrusion, the magnesium tube is annealed at 300° C. for 6hours. This annealing time leads to a uniform, fine-grained structure.One who is versed in the art will appreciate that a fine grain structureis particularly suitable for IHPF and similar processes. Thereafter, themagnesium tubes are preheated to a forming temperature of 350° C. Theexpansion zone is kept at a temperature of 350° C., and the guided zoneis kept at a temperature of 250° C. At 350° C., the total elongationallowed is near a peak of about 35%. An axial force is applied by thehydraulic cylinders, which causes a compressive stress condition in thewall of the tube. As the pressure medium, nitrogen gas is used. Thepressure vs. time path ensures a constant strain rate over the processtime.

1. An article made of a magnesium alloy tube, the article having a grainsize of between 10 μm and 50 μm and being manufactured by internal highpressure forming.
 2. An article according to claim 1, wherein thetemperature of the internal high pressure forming is between 200° C. and605° C.
 3. An article according to claim 1, wherein the tube wasmanufactured by extrusion.
 4. An article according to claim 3, whereinthe extrusion temperature is between 300° C. and 605° C.
 5. An articleaccording to claim 3, wherein the extrusion speed is substantiallybetween 5 mm/sec and 45 mm/sec.
 6. An article according to claim 3,wherein the extrusion reduction ratio is substantially between 10:1 and50:1.
 7. An article according to claim 3, wherein the extrusiontemperature is between 300° C. and 605° C., the extrusion speed issubstantially between 5 mm/sec and 45 mm/sec, and the extrusionreduction ratio is substantially between 10:1 and 50:1.
 8. An articleaccording to claim 1, wherein the magnesium allow is selected from thegroup consisting of AZ31 and ZM21.
 9. An article according to claim 8,wherein the extrusion reduction ratio is substantially 30:1, theextrusion speed is substantially 15 mm/sec, the predeterminedtemperature is substantially 300° C., and the AZ31 alloy is used.
 10. Anarticle according to claim 1, wherein the magnesium alloy comprises2.856% aluminum, 1.022% zinc, 0.329% manganese, 0.004% iron, 0.038%silicon, 0.001% copper, and 0.001% nickel.
 11. An article according toclaim 1, wherein the tube is annealed.
 12. An article according to claim11, wherein the tube is annealed at a temperature of 300° C. for sixhours.
 13. A process for manufacture of a tube from a billet made of amagnesium alloy, the process comprising: (a) heating the billet to apredetermined temperature that is within a range of 300° C. to 605°; (b)extruding the billet, using an extrusion press having a ram, an internalpiercing mandrel, and a die, while maintaining the temperature of thebillet to stay within the range; and (c) applying a force to the billetso that it is forced between the die and the mandrel at a predeterminedextrusion speed of the ram to form a tube having a predeterminedextrusion reduction ratio; wherein the extrusion speed is substantiallybetween 5 mm/sec and 45 mm/sec, and the extrusion reduction ratio issubstantially between 10:1 and 50:1.
 14. A process according to claim13, wherein the magnesium alloy is selected from the group consisting ofAZ31 and ZM21.
 15. A process according to claim 14, wherein theextrusion reduction ratio is substantially 30:1, the extrusion speed issubstantially 15 mm/sec, the predetermined temperature is substantially300° C., and the AZ31 alloy is used.
 16. A process according to claim 1,wherein the magnesium alloy comprises 2.856% aluminum, 1.022% zinc,0.329% manganese, 0.004% iron, 0.038% silicon, 0.001% copper, and 0.001%nickel.
 17. A process according to claim 1, wherein the process furthercomprises the step of annealing the tube.
 18. A process according toclaim 17, wherein the tube is annealed at a temperature of 300° C. forsix hours.
 19. A process according to claim 13, further comprising thesteps of (d) cooling the tube at a predetermined first coolingtemperature for a predetermined amount of time; (e) sealing the tubefrom both ends; (f) introducing a pressure medium into the tube; (g)positioning the tube in a mold having a guiding zone at a predeterminedguiding temperature and an expansion zone of a predetermined shape at apredetermined expansion temperature; (h) applying an axial compressionforce on the tube so that a section of the tube located in the expansionzone expands to conform to the predetermined shape; and (i) cooling thetube at a predetermined second cooling temperature for a predeterminedamount of time; wherein the expansion temperature is within a range of200° C. to 605°.
 20. A process according to claim 19, wherein theexpansion temperature is substantially between 200° C. and 500° C.
 21. Aprocess according to claim 19, wherein the pressure medium is a gas. 22.A process according to claim 21, wherein the gas does not react with themetal under the conditions used in the process.
 23. A process accordingto claim 19, wherein the pressure medium is a heat resistant liquid.